Direct Current (DC) power transmission systems are of interest to use in various situations, for instance when transferring electrical power over long distances.
High Voltage Direct Current (HVDC) transmission of power has proved to be an efficient alternative to Alternating Current (AC) transmission in many power transmission situations. Several HVDC transmission lines may furthermore be interconnected to form an HVDC transmission network.
These power systems are typically equipped with converters for converting between AC and DC. Furthermore, in these systems the power levels keep getting higher.
With the increase in the power levels being transmitted over HVDC systems, a need to enhance the power rating of the HVDC converters has arisen. This can be achieved by increasing the voltage or current levels of the converter. However, semiconductor devices used in the converter are often being operated at their maximum voltage and current limits.
Another way of increasing the power rating of a converter is to connect more semiconductors, arms or converters in parallel. Connecting switches in parallel needs complicated Gate Drivers with more focus on the physical circuit layout to synchronize their switching. Hence, paralleling of arms/converters is often an attractive option.
The use of a converter with parallel converter arms is described in US 2008/0252142.
One control scheme for controlling such a converter with parallel converter arms is described by Pou, Josep; Ceballos, Salvador; Konstantinou, Georgios; Capella, Gabriel J.; Agelidis, Vassilios G., in “Control strategy to balance operation of parallel connected legs of modular multilevel converters”, Industrial Electronics (ISIE), 2013 IEEE International Symposium, 28-31 May 2013.
Under ideal conditions, load current should be distributed equally between both the parallel arms. However, even a small mismatch between the parameters of both the arms leads to a flow of circulating current in the converter. This current can flow between the two parallel arms, on the AC side or on the DC side of the converter. This causes an imbalance in the distribution of load current between different phases leading to additional losses or instability in the system. This is one of the important issues in paralleling of converter arms. Hence, there is a need to develop a control to limit the circulating current flowing between the parallel connected converter arms.